Process and apparatus for air separation

ABSTRACT

A first process stream of the air separation is compressed in a first compressor ( 2 ). The first compressor ( 2 ) is mechanically coupled ( 4 ) to a steam turbine ( 1 ) and to an electric motor ( 3 ).

[0001] The invention concerns a process for air separation, wherein feedair is fed to an air separation unit, and a first process stream of theair separation is compressed in a first compressor. Such firstcompressor is driven by the energy produced by work expansion of steamin a steam turbine, whereby such energy is transferred by mechanicalcoupling between such turbine and the main air compressor.

[0002] “Air separation” can be any type of physical separation processfor producing at least one air gas, e.g., nitrogen and/or oxygen, fromatmospheric air. Even though the invention is equally applicable toadsorption and membrane processes, it primarily refers to cryogenicprocesses separating air by distillative methods. Such cryogenicprocesses are known in general from the monograph“Tieftemperaturtechnik” of Hausen/Linde (2^(nd) ed., 1985) or from apaper of Latimer in Chemical Engineering Progress (vol. 63, no.2, 1967,p. 35), the disclosures of which are hereby incorporated by reference.

[0003] Simple air separation units are also described in Perry'sChemical Engineer's Handbook, 6^(th) Edition (1984), at pages 12-53 and12-54. For example, in the Linde Single-Column System for airseparation, feed air is compressed in an air compressor and then cooledin a heat exchanger. In the heat exchanger, the compressed feed air issubjected to heat exchange with, for example, waste nitrogen and/orproduct oxygen gas. Thereafter, the compressed feed air is sent througha coil in the boiler of a rectification column wherein it is furthercooled. The feed air is then expanded, whereby the air becomes mostlyliquid, and introduced to the top of the rectification column. Nitrogengas and gaseous or liquid oxygen product are removed from therectification column. See FIG. 12-67 in Perry's. In the LindeDouble-Column System for air separation (illustrated at FIG. 12-68 inPerry's), two rectification columns are placed on top of one another.The compressed feed air, after being subjected to heat exchange withnitrogen and oxygen gas streams, is sent through a coil in the boiler ofthe lower column ands then expanded and introduced at an intermediatelocation into the lower column. Nitrogen is removed from the top of thebottom column and delivered to the top of the upper column. In addition,oxygen rich-liquid from the bottom boiler of the lower column isintroduced at an intermediate point into the upper column. Pure oxygenis then removed from the bottom of the upper column and pure nitrogen isremoved from the top thereof.

[0004] A “process stream of the air separation” can be formed by any ofthe following streams: feed air (total or portion) for the airseparation, product of the air separation, recycle stream in the airseparation. That means that the first compressor may be operated, e.g.,as a main air compressor, booster air compressor, or product compressor.It comprises one or more stages with or without intercooling. If thefirst compressor is a multiple stage compressor, it may compress morethan one process stream of the air separation.

[0005] “Mechanical coupling” can be realized as direct coupling by acommon shaft or as indirect coupling by a gear box transmission.

[0006] The preferred field of application of the invention are airseparators combined with Integrated Gasification Combined Cycle (IGCC)processes, Gas to Liquid (GTL), or other steam producing clientprocesses, e.g. steel works. Such processes require large amounts ofoxygen for their operation, which are normally produced by cryogenic airseparation. One of the products of these processes is pressurized steam,which often is not or not completely used in the client process. Theeconomics of these combined processes depends on the efficient use ofthe available steam. The practice has been to use the steam for the airseparation plant compressors. The steam available from such processesoften varies in its energy content, i.e. it is too low in pressureand/or temperature (low-grade steam) at least in some periods ofoperation. The steam may not contain enough energy to drive thecompressor at least in such periods.

[0007] A process of the kind described above is known from E.Schönpflug, Linde Reports on Science and Technology 23/1976, p. 55, thedisclosure of which is hereby incorporated by reference. In thisexample, a steam turbine driven air compressor and an electricallydriven air compressor are operated in parallel. This configurationovercomes the problem of missing steam power. It requires, however, hugeapparatus and control effort.

SUMMARY OF THE INVENTION

[0008] An object of the invention is, to provide a different concept,which reduces effort for apparatus and/or control, and/or is moreflexible in adaptation to operating conditions.

[0009] Upon further study of the specification and appended claims,further objects and advantages of this invention will become apparent tothose skilled in the art.

[0010] These objects are achieved by an air separation processcomprising feeding feed air into an air separation unit, and compressinga first process stream of the air separation unit in a first compressor(2) which is mechanically coupled (4) to a steam turbine (1) and to anelectric motor (3).

[0011] The three machines, turbine driven by the external fluid, firstcompressor, frequently operated as main air compressor, and electricmotor can be mounted on a common shaft or mechanically connected byother means, e.g., by one or more gear boxes. It is possible to couplejust those three machines, or to include further machines into themechanical coupling, e.g., a further compressor and/or a gas expander.In the process of the invention, the mechanical energy provided by theexternally driven turbine can be supplemented, at least in certainoperating periods, by electric energy driving the electric motor. Bythis measure, a wide range of operating conditions, particularlyconcerning steam availability, can be covered, whereby in any case, thecomplete steam power available for compression requirements of the airseparator can be used for such purpose.

[0012] If the amount of steam produced by the plants varies, the motoris then sized to match the power deficit appearing in some operationmode.

[0013] A single steam turbine drive and single condenser are used. Thesize of the steam turbine may be up to the maximum available or up tothe maximum the end user will accept from a technical risk. A specificapplication of the invention are processes which produce relatively lowsteam pressures and temperatures (e.g. 15 bara-25 bara and 200° C.-350°C. in case of GTL). This would normally require the use of largemechanical drive turbines, which does. These turbines either do notexist or pose a technical risk. If a mechanical steam turbine drivelarge enough for the compressor does not exist (currently the largestreferenced mechanical drive steam turbine is 57 MW the invention enablesthe difference in power required to be made up by the motor.

[0014] A further advantage of the invention can be used during start-up.Starting compressors with a steam turbine requires rapid acceleration inorder to pass quickly through resonant frequencies (critical speeds). Inthe invention, the electric motor drive is able to assist in startingand the acceleration is increased.

[0015] Alternatively, if the electric motor is large compared with theelectrical system network then the machine train can be run up to speedwith the motor power off. Once at full speed the motor is started. Thisgives the advantage of eliminating expensive starting equipment. Themotor is direct-on-line (DOL) started.

[0016] Even though the invention is quite advantageous for systems,where the steam power is too low for all operating conditions, so thatalways electric energy is supplied to the electric motor, additionalflexibility can be gained by using a motor/generator as electric motorand operating it during some operating period as a generator.

[0017] In many cases, the air separation process comprises thecompression of a second process stream of the air separation in a secondcompressor, frequently operated as booster compressor, in addition tothe first compressor, frequently operated as main air compressor. Thesecond process stream can be constituted by a portion of the feed airdownstream the main air compressor, which is further compressed in orderto be used for evaporation of an internally compressed stream of aproduct. Alternatively it may be a product from the air separator (e.g.,at least a part of the oxygen or nitrogen product produced in the airseparation), which is to be compressed to a delivery pressure; in thiscase the booster compressor acts as a product compressor. In anotherexample, the second process stream may be constituted by a recyclestream of the air separation. According to a further aspect of theinvention, the second compressor can be driven as well by theturbine-motor combination.

[0018] The second compressor comprises one or more stages with orwithout intercooling. Normally, but not necessarily, the secondcompressor is smaller in capacity than the first compressor.

[0019] The steam turbine is directly coupled to one of the airseparation plant compressors whilst the other air separation plantcompressor is connected either directly to the other side of the steamturbine or via a step up/down gear or integrated compressor gear. Theadditional power required by the train is supplied by the electric motoror, if surplus power is available, the motor/generator is operated as agenerator.

[0020] In certain cases, the first and second compressors can bedirectly coupled. If the mechanical coupling between the secondcompressor and the first compressor is realized by a gear system, thegear is able to match for different speeds of the first compressor andthe second compressor.

[0021] Furthermore, the invention also relates to an apparatus for airseparation comprising a first compressor (2) and means for connectingthe inlet or the outlet of the first compressor to an air separationunit, wherein the first compressor (2) is mechanically coupled to asteam turbine (1) and to an electric motor (3).

[0022] The invention also relates to an apparatus for compressing andexpanding fluids comprising a first compressor (2), a steam turbine (1),a second compressor (5), and a electric motor (3), all beingmechanically coupled (4 a, 4 b, 6). This last apparatus can be used inany application, not only in connection with an air separation unit.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Various other features and attendant advantages of the presentinvention will be more fully appreciated as the same becomes betterunderstood when considered in conjunction with the accompanyingdrawings, in which like reference characters designate the same orsimilar parts throughout the several views, and wherein:

[0024]FIG. 1 shows a basic implementation of the invention; and

[0025]FIG. 2 is a more specific embodiment.

[0026] In the specific embodiment of FIG. 1, the external fluid to bework expanded is steam. The machine train comprises a single steamturbine 1, a single multiple stage main air compressor (“firstcompressor”) 2 and an electric motor 3. All three machines are directlymechanically coupled by a common shaft 4. In operation, they always runat the same speed. (Such direct coupling is always preferred, if thespeeds of the machines match. Nevertheless, as an alternative, any ofthe couplings 4 can be realized by a gear box as well.)

[0027] In a particular example, the inlet of the main air compressor 2is connected to a source for atmospheric air. The air compressed in themain air compressor is purified, cooled and fed to a cryogenic airseparation unit having at least one distillation column. The airseparation unit produces impure oxygen, which is delivered to an oxygenconsuming process, which produces the steam to be expanded in turbine 1.

[0028] Alternatively, a motor/generator is used as electric motor 3. Inthis way, surplus steam power available during certain operation periodscan be converted into electric energy.

[0029] The embodiment of FIG. 2 is particularly suited for the case,where the air separation has more than one compression duty. In additionto the steam turbine 1, the main air compressor 2 and a motor/generator3, which are known from FIG. 1, this machine train comprises a boostercompressor (“second compressor”) 5 for compressing an oxygen or nitrogenstream produced in the air separation unit. Two shafts 4 a, 4 b effect adirect mechanical coupling between main air compressor 2 and turbine 1on the one hand, and booster compressor 5 and motor 3 on the other hand.Those pairs of machines are connected by an indirect mechanical couplingeffected by a gear-box 6.

[0030] There are a lot of variations possible in the invention. Forinstance, depending on the particular circumstances of a specific plant,any of the couplings shown in the drawings may be direct (e.g. by acommon shaft), or indirect (e.g. by a gear-box). Normally one tends tominimize or totally avoid gear-boxes in order to reduce investmentcosts.

[0031] In special cases, the first compressor and/or the secondcompressor of the invention can be realized as one or two combinedservice machines, each performing more than one compression duty. Forexample, several stages of the first compressor may act as a main aircompressor for feed air, whilst one or more other stages are operated asbooster air compressor and/or product compressor.

[0032] The entire disclosures of all applications, patents andpublications, cited above and below, and of corresponding EuropeanApplication No. 00122168.8, filed Oct. 12, 2000 are hereby incorporatedby reference.

[0033] From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention and, withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

1. In a process for air separation comprising feeding air in, whereinfeed air is fed to an air separation unit, and compressing a firstprocess stream of the air separation is compressed in a first compressor(2), the improvement wherein said first compressor isbeing mechanicallycoupled (4) to a steam turbine (1) driven by an external fluid and to anelectric motor (3).
 2. A process according to claim 1, wherein saidfirst compressor is the main feed air compressor of said air separationunit.
 3. A process according to claim 1, wherein said first compressoris a booster air compressor of said air separation unit.
 4. A processaccording to claim 1, wherein said first compressor is a product gascompressor of said air separation unit.
 5. A process according to claim1, wherein said a first process stream is feed air for said airseparation unit.
 6. A process according to claim 1, wherein said a firstprocess stream is a product gas of said air separation unit.
 7. Aprocess according to claim 1, wherein said a first process stream is arecycle gas of said air separation unit.
 8. A process according to claim1, wherein during at least a first operating period, electric energy issupplied to the motor.
 9. A process according to claim 1, wherein theelectric motor (3) is a motor/generator and wherein during at least asecond operating period, electric energy is withdrawn from themotor/generator.
 10. A process according to claim 2, wherein theelectric motor (3) is a motor/generator and wherein during at least asecond operating period, electric energy is withdrawn from themotor/generator.
 11. A process according to claim 1, wherein a secondprocess stream of the air separation is compressed in a secondcompressor (5) and the second compressor is mechanically coupled to thefirst compressor (2).
 12. A process according to claim 11, wherein themechanical coupling between the second compressor (5) and the firstcompressor (2) is realized by a gear system (6).
 13. In an apparatus forair separation comprising a first compressor (2) and means forconnecting the inlet or the outlet of the first compressor to an airseparation unit, the improvement wherein the first compressor (2) ismechanically coupled to a steam turbine (1) for work expansion of afluid and to an electric motor (3).
 14. An apparatus according to claim13, wherein the electric motor (3) is a motor/generator.
 15. Anapparatus according to claim 13, further comprising a second compressor(5) which is mechanically coupled to the first compressor (2).
 16. Anapparatus according to claim 14, further comprising a second compressor(5) which is mechanically coupled to the first compressor (2).
 17. Anapparatus according to claim 15, wherein the mechanical coupling betweenthe second compressor (5) and the first compressor (2) is realized by agear system (6).
 18. An apparatus according to claim 16, wherein themechanical coupling between the second compressor (5) and the firstcompressor (2) is realized by a gear system (6).
 19. An apparatus forcompressing and expanding fluids, comprising a first compressor (2), asteam turbine (1), a second compressor (5), and a electric motor (3),all being mechanically coupled (4 a, 4 b, 6).
 20. An apparatus accordingto claim 19, wherein the first compressor (2) and the turbine (1) aremechanically coupled by a common shaft (4 a), the second compressor (5)and the electric motor (3) mechanically coupled by a common shaft (4 b),and the compressor-turbine combination and the compressor-motorcombination are mechanically coupled via a gear system (6). 21.Apparatus according to claim 19, whereby the electric motor (3) is amotor/generator.
 22. Apparatus according to claim 20, whereby theelectric motor (3) is a motor/generator.